PRUDP

PRUDP is a third-party network protocol used for network communications by some Nintendo software on the 3DS. It presumably stands for "pretty reliable UDP" or "partially reliable UDP" or some such, and essentially implements TCP-like reliability via sequence numbers and acknowledgement within UDP. Optionally it also adds security (prudps). This is probably used for online comms in most(?) 3DS games.

At least two different versions of PRUDP seem to be used by Nintendo software: an older version used in the Friend services and a newer one used in Pokémon X/Y.

Version 1

This is the version of the protocol used by the Friend services.

Packet structure

Type	Name	Description
u16	src_dst	Magic: client-to-server (afa1) or server-to-client (a1af)
u16	op	Type of packet
u8	unk1
u32	unk2
u16	seqno	Incrementing ID to pair up data packets and acknowledgement
u8[*]	payload
u8	checksum	seems to be related to the sum of the rest of the fields (mod 256)

Different kinds of packets have different values for the different fields:

• Handshake (1): op=0x40 or op=0x10 (cli-to-serv or serv-to-cli)
  • unk1, unk2, seqno are 0
  • payload: u32 unk3;

• Handshake (2): op=0x61 or op=0x11
  • unk1, seqno are 0
  • payload: u32 unk3;

• Hangup: op=0x63 or op=0x13

• Heartbeat: op=0x44 or op=0x14
  • seqno = increasing

• Data packet: op=0x62
  • seqno = increasing
  • payload: u8[?] payload;

• Data ack: op=0x12
  • unk2 = 0x12345678
  • seqno = corresponding to the data packet being acknowledged

Sample session

Handshake 1   → af a1 40 00 00 00 00 00 00 00 00 00 00 00 00 97
Handshake 1   ← a1 af 10 00 00 00 00 00 00 00 00 5f 22 68 ea 3a
Handshake 2   → af a1 61 00 18 5f 22 68 ea 01 00 d4 d6 91 e8 c9
Handshake 2   ← a1 af 11 00 50 d4 d6 91 e8 01 00 00 00 00 00 de
Send data     → af a1 62 00 18 26 b4 01 a1 02 00 00 (25 bytes of encrypted payload) ef
Acknowledge   ← a1 af 12 00 50 78 56 34 12 02 00 00 d1
Send data     ← a1 af 62 00 50 67 dd f9 c3 01 00 00 (255 bytes of encrypted payload) 03
Acknowledge   → af a1 12 00 18 78 56 34 12 01 00 00 97
Send data     → af a1 62 00 18 8d 58 91 c0 03 00 00 (21 bytes of encrypted payload) fa
Acknowledge   ← a1 af 12 00 50 78 56 34 12 03 00 00 d2
Send data     ← a1 af 62 00 50 a9 c5 fa 2e 02 00 00 (130 bytes of encrypted payload) 54
Acknowledge   → af a1 12 00 18 78 56 34 12 02 00 00 98
Hangup        → af a1 63 00 18 5f 22 68 ea 04 00 aa
Hangup        ← a1 af 13 00 50 d4 d6 91 e8 04 00 e2

During the handshake process, the unk2 and unk3 fields exhibit a pattern where the unk3 of the preceding packet is used as the unk2 of the next one. The parties might be agreeing upon a key using a key-exchange scheme?

Payload

The encrypted payload consists of a small header that looks like:

Type	Name	Description
u32	length	Length of the remaining payload (excluding this field)
u8	op	Type of packet
u8	unk1	(Only for server-to-client messages, seemingly always 1)
u32	msgid	Seemingly used to pair up requests and responses
u32	unk2

The encrypted payloads exchanged when first connecting look something like (excluding the header):

→ string deviceID;
← u16 unk1; u16 unk2; u32 deviceID; u32 length; u8[length] blob; string connectionString; u8[7] unk3; string buildString;
→ u32 deviceID; u32 unk1;
← u16 unk1; u16 unk2; u32 length; u8[length] blob;

where string is both length-prefixed (u16) and zero-terminated (the terminating zero is not included in the length).

The connectionString looks like 'prudps:/address=203.180.85.79;port=50051;CID=1;PID=2;sid=1;stream=10;type=2' and the buildString is always 'branch:unknown build:2_24_13767_0'. The deviceID seems to be unique to each 3DS or perhaps online account (it seems to roughly scale with the age of the 3DS, so that an older 3DS sends a lower ID).